Circuit arrangements for starting and operating discharge lamps are used in electronic operating devices for discharge lamps. The starting of the discharge lamp is understood hereafter as meaning at least the ignition during an igniting phase. However, this may also be preceded by a preheating of electrode filaments during a preheating phase of the igniting phase. If the operating devices are operated on a line voltage, they have to conform to relevant regulations with respect to line current harmonics, for example IEC 1000-3-2. To ensure compliance with these regulations, circuit measures are necessary for reducing line current harmonics. Such a measure is the installation of so-called charge pumps. The advantage of charge pumps is the low level of circuit complexity necessary to realize them.
Circuit arrangements for operating discharge lamps which are operated on a line voltage generally include the following elements:                a rectifier for rectifying the line voltage        a main energy store        an inverter, which draws energy from the main energy store and produces at an inverter output an inverter voltage which has an inverter frequency that is much higher than the line frequency        a matching network, via which discharge lamps can be coupled to the inverter output.        
If the main energy store is charged directly from the rectifier, this produces charge current peaks, which lead to infringement of said regulations.
The topology of a charge lamp comprises that the rectifier is coupled to the main energy store via an electronic pumping switch. As a result, a pumping node is produced between the rectifier and the electronic pumping switch. The pumping node is coupled to the inverter output via a pumping network. The pumping network may include components which can at the same time be assigned to the matching network. The principle of the charge pump is that, during a half-period of the inverter frequency, energy is drawn from the line voltage via the pumping node and buffer-stored in the pumping network. In the half-period of the inverter frequency which then follows, the buffer-stored energy is fed via the electronic pumping switch to the main energy store.
Accordingly, energy is drawn from the line voltage in time with the inverter frequency. The electronic operating device generally includes filter circuits, which suppress spectral components of the line current lying at or above the inverter frequency. The charge pump may be designed in such a way that the harmonics of the line current are low enough to comply with said regulations. The following documents provide a detailed description of charge pumps for electronic operating devices for discharge lamps:
Qian J., Lee F. C., Yamauchi, T.: “Analysis, Design and Experiments of a High-Power-Factor Electronic Ballast”, IEEE Transactions on Industry Applications, Vol. 34, No. 3, May/June 1998
Qian J., Lee F. C., Yamauchi, T.: “New Continuous Current Charge Pump Power-Factor-Correction Electronic Ballast”, IEEE Transactions on Industry Applications, Vol. 35, No. 2, March/April 1999.
In the document EP 0 621 743 (Mattas) there is a description of a circuit arrangement for operating a discharge lamp which includes a charge pump. It additionally has a controller which brings about a modulation of the inverter frequency with twice the line frequency. This achieves the object of improving the crest factor of the lamp current that is applied to the discharge lamp. The service life of the lamps is consequently increased.
The aforementioned matching network includes a resonant circuit, which essentially includes a resonant capacitor and a lamp inductor. The resonant circuit has a resonant frequency, which, without damping of the resonant circuit, lies at a natural frequency of the resonant circuit.
For igniting the discharge lamp, the inverter is initially operated at an inverter frequency that lies above the natural frequency. In an igniting phase, the inverter frequency is lowered until it is close to the natural frequency of the resonant circuit, generates a high voltage at the discharge lamp and ignites the discharge lamp.
In this case, the following problem occurs: before the igniting of the discharge lamp, on the one hand there is no significant energy consumer in the circuit arrangement. On the other hand, the charge pump is operating and constantly depositing energy in the main energy store. This produces an imbalance between the energy received by the circuit arrangement and the energy delivered by it. If the discharge lamp does not ignite promptly, this leads either to the main energy store being destroyed or to the circuit arrangement being switched off, if switching-off means are provided for this purpose.
In the prior art, this leads to an optimization problem for the choice of the inverter frequency during the igniting phase: On the one hand, the time in which said energy imbalance prevails is to be short. This achieves a high ignition voltage, which demands an inverter frequency close to the natural frequency. On the other hand, the energy imbalance is to be as small as possible, in order that the time to overloading of the main energy store, and consequently the igniting phase, can be as long as possible. This is desirable for reliable ignition of the discharge lamp, but demands an inverter frequency that lies as far as possible above the natural frequency. The optimizing task is made more difficult by the fact that external circumstances, such as for example the igniting properties of the discharge lamp, ambient temperature and component tolerances, have an influence on it.
In the prior art, there are two solutions to the problem: either unreliable ignition of the discharge lamp is accepted, or components such as the main energy store and lamp inductor are overdimensioned, and consequently become expensive and bulky.